Definition
The plasmasphere is a region of the Earth's magnetosphere filled with relatively cold, dense plasma that co-rotates with the planet. It extends outward from the ionosphere into the inner magnetosphere and is bounded by a sharp gradient in plasma density known as the plasmapause.
Overview
The plasmasphere occupies the inner portion of the magnetosphere, typically ranging from the Earth's surface to distances of about 4–6 Earth radii (Rₑ) on the dayside and up to 8–10 Rₑ on the nightside during periods of low geomagnetic activity. The plasma within this region is mainly composed of ionized hydrogen (protons) and helium ions that originate from the topside ionosphere and are transported upward by centrifugal forces and magnetic field line diffusion. The plasmasphere plays a crucial role in the dynamics of the magnetosphere, influencing phenomena such as wave propagation, particle precipitation, and the formation of radiation belts.
Etymology/Origin
The term combines “plasma,” referring to an ionized gas of freely moving charged particles, with “sphere,” indicating its roughly spherical shape surrounding the Earth. The concept was introduced in the mid‑20th century as scientists began to distinguish between the hotter, more tenuous plasma of the outer magnetosphere and the cooler, denser plasma nearer Earth.
Characteristics
- Composition: Predominantly H⁺ (protons) with smaller fractions of He⁺ and O⁺ ions; electron densities range from 10⁴ cm⁻³ near the ionosphere to about 10 cm⁻³ near the plasmapause.
- Temperature: Electron temperatures are typically 1,000–2,000 K, considerably lower than the several keV temperatures found in the outer magnetosphere.
- Co‑rotation: Magnetic field lines enforce near‑rigid co‑rotation with the Earth, causing the plasma to move at angular velocities close to the planetary rotation rate.
- Plasmapause: A sharp boundary where plasma density drops by one to two orders of magnitude; its location varies with geomagnetic activity, moving earthward during storms and outward during quiet periods.
- Dynamics: The plasmasphere is subject to refilling (during quiet times) and erosion (during geomagnetic storms) processes, driven by electric fields, wave–particle interactions, and convection patterns.
- Observational Methods: Satellite plasma probes (e.g., the Defense Meteorological Satellite Program, Van Allen Probes), ground‑based riometers, and whistler‑mode wave analyses have been used to map its structure and temporal variations.
Related Topics
- Ionosphere: The atmospheric layer below the plasmasphere that provides the source plasma.
- Magnetosphere: The larger region of space dominated by Earth's magnetic field, of which the plasmasphere is the inner component.
- Plasmapause: The defining boundary separating the plasmasphere from the outer plasma environment.
- Radiation belts: Zones of high‑energy charged particles whose dynamics are influenced by the plasmaspheric plasma.
- Whistler-mode waves: Electromagnetic waves that propagate through the plasmasphere and affect electron dynamics.
- Geomagnetic storms: Disturbances that can compress or erode the plasmasphere, altering its size and density.